Imaging device

ABSTRACT

In one general aspect, the techniques disclosed here feature an imaging device that includes: a semiconductor substrate; a first pixel cell including a first photoelectric converter in the semiconductor substrate, and a first capacitive element one end of which is electrically connected to the first photoelectric converter; and a second pixel cell including a second photoelectric converter in the semiconductor substrate. An area of the second photoelectric converter is larger than an area of the first photoelectric converter in a plan view.

BACKGROUND

1. Technical Field

The present disclosure relates to an imaging device and an imagingmodule, which are typified by complementary metal-oxide semiconductor(CMOS) image sensors.

2. Description of the Related Art

In the natural world, subjects that have high contrasts exist. Forexample, for dealing with a subject whose brightness changes from momentto moment, vehicle-mounted imaging devices need to simultaneously imagea bright subject and a dark subject (i.e., need to have a high dynamicrange). In order to realize a high dynamic range, Japanese UnexaminedPatent Application Publication No. 62-108678 (hereinafter referred to as“Patent Document 1”) and Japanese Unexamined Patent ApplicationPublication No. 2008-99073 (hereinafter referred to as “Patent Document2”) propose methods as described below.

Imaging devices disclosed in Patent Documents 1 and 2 use siliconphotodiodes. In Patent Document 1, images for which exposure times(hereinafter may be referred to as “storage times”) differ from eachother are combined together to thereby make it possible to obtain a widedynamic range. This scheme has already been put into practical use. InPatent Document 2”, images acquired from pixel cells that are arrangedin one pixel and that have different sensitivities are combined togetherto increase the dynamic range.

SUMMARY

The above-described imaging device in the related art requires a furtherimprovement in high-dynamic-range photography.

In one general aspect, the techniques disclosed here feature an imagingdevice that includes: a semiconductor substrate; a first pixel cellincluding a first photoelectric converter in the semiconductorsubstrate, and a first capacitive element one end of which iselectrically connected to the first photoelectric converter; and asecond pixel cell including a second photoelectric converter in thesemiconductor substrate. An area of the second photoelectric converteris larger than an area of the first photoelectric converter in a planview.

It should be noted that general or specific embodiments may beimplemented as a system, a method, an integrated circuit, a computerprogram, a storage medium, or any selective combination thereof.

Additional benefits and advantages of the disclosed embodiments willbecome apparent from the specification and drawings. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram schematically illustrating pixel cellcharacteristics in the related art and desirable pixel cellcharacteristics;

FIG. 2 is a diagram schematically illustrating the pixel cellcharacteristics in the related art and more desirable pixel cellcharacteristics;

FIG. 3 is a block diagram schematically illustrating one example of thestructure of an imaging device according to an exemplary firstembodiment;

FIG. 4 is a circuit diagram of one unit pixel in the exemplary firstembodiment;

FIG. 5 is a sectional view schematically illustrating the devicestructure of each unit pixel in the imaging device according to theexemplary first embodiment;

FIG. 6 is a sectional view schematically illustrating another devicestructure of each unit pixel in the imaging device according to theexemplary first embodiment;

FIG. 7 is a sectional view schematically illustrating another devicestructure of each unit pixel in the imaging device according to theexemplary first embodiment;

FIG. 8 is a sectional view schematically another device structure ofeach unit pixel in the imaging device according to exemplary firstembodiment;

FIG. 9 is a sectional view schematically illustrating yet another devicestructure of each unit pixel in the imaging device according to theexemplary first embodiment;

FIG. 10 is a layout diagram of the unit pixels in the imaging deviceaccording to the exemplary first embodiment when they are viewed from abird's eye;

FIG. 11 is a circuit diagram schematically illustrating a variation ofeach unit pixel according to the exemplary first embodiment;

FIG. 12 is a circuit diagram schematically illustrating a variation ofeach unit pixel according to the exemplary first embodiment;

FIG. 13 is a circuit diagram schematically illustrating a variation ofeach unit pixel according to the exemplary first embodiment;

FIG. 14 is a circuit diagram schematically illustrating a variation ofeach unit pixel according to the exemplary first embodiment;

FIG. 15 is a circuit diagram schematically illustrating a variation ofeach unit pixel according to the exemplary first embodiment;

FIG. 16 is a circuit diagram schematically illustrating a variation ofeach unit pixel according to the exemplary first embodiment;

FIG. 17 is a circuit diagram schematically illustrating a variation ofeach unit pixel according to the exemplary first embodiment;

FIG. 18 is a circuit diagram schematically illustrating a variation ofeach unit pixel according to the exemplary first embodiment;

FIG. 19 is a circuit diagram schematically illustrating a variation ofeach unit pixel according to the exemplary first embodiment;

FIG. 20 is a circuit diagram schematically illustrating a variation ofeach unit pixel according to the exemplary first embodiment;

FIG. 21 is a circuit diagram schematically illustrating a variation ofeach unit pixel according to the exemplary first embodiment;

FIG. 22 is a circuit diagram schematically illustrating a variation ofeach unit pixel according to the exemplary first embodiment;

FIG. 23 is a timing chart illustrating a timing of exposure and a readoperation in one cycle (one frame) period in the imaging deviceaccording to the exemplary first embodiment; and

FIG. 24 is a diagram schematically illustrating functional blocks in animaging module including the imaging device.

DETAILED DESCRIPTION

First, a description will be given of problems in the related art whichare considered by the present inventors.

In the image combination disclosed in Patent Document 1, a plurality ofpieces of image data are obtained in chronological order. Thus, a periodof time that is a few times as long as a typical imaging time isrequired in order to acquire one combined image. In addition, sinceimages having time differences are combined together, simultaneity ofimages is impaired, thus causing disturbance in images of movingsubjects.

In Patent Document 2, a plurality of photodiodes having the same numberof saturation electrons and the same size is used. An on-chip top lensis provided to divide the amount of light that is incident on each ofthe photodiode into two types, that is, a large amount of light and asmall amount of light. This configuration can make effectivesensitivities appear be different from each other between the pixelcells. Since two pixel cells are provided in one pixel, simultaneousimaging is made possible to ensure the simultaneity of images.

However, since two cells need to be provided in one pixel, the area ofeach photodiode inevitably becomes one-half or less of that in typicaltechnologies. The area of each photodiode is generally proportional tothe sensitivity or the number of saturation electrons. As a result, whenthe area of each photodiode is one-half or less, the sensitivity and thenumber of saturation electrons also become one-half or less of the areain typical technologies.

FIG. 1 schematically illustrates pixel cell characteristics in therelated art and desirable pixel cell characteristics. As opposed to atypical cell having one pixel cell in a single pixel (hereinafter simplyreferred to as a “typical cell”), two pixel cells in a single pixel areused in high dynamic range (HDR) photography. It is desirable that eachof these two pixel cells has (a) pixel cell characteristics in which thesensitivity and the number of saturation electrons are equivalent tothose of the typical cell and (b) pixel cell characteristics in whichthe number of saturation electrons is equivalent to that of the typicalcell and the sensitivity is lower than that of the typical cell. In FIG.1, “a” and “b” represent this desirable combination.

Also “a” and “b” in FIG. 1 represent a combination of two pixel cells inPatent Document 2. As described above, the area of each pixel cell(photodiode) is one-half or less of the typical cell. Accordingly, thesensitivity of each pixel cell decreases, and the number of saturationelectrons decreases. This means that the characteristics deviate fromthe desirable characteristics. As described above, the characteristicsof the pixel cells in Patent Document 2 are significantly inferior tothe requested characteristics.

FIG. 2 schematically illustrates the pixel cell characteristics in therelated art and more desirable pixel cell characteristics. Asrepresented by “b” in FIG. 2, saturation that can occur when the amountof incident light is large is reduced by reducing the sensitivity. Inaddition, when the number of saturation electrons can be increased, thedynamic range increases further.

Embodiments according to the present disclosure will be described belowwith reference to the accompanying drawings. The present disclosure,however, is not limited to the embodiments. Changes can be made asappropriate without departing from the scope in which advantages of thepresent disclosure are obtained. In addition, one embodiment can also becombined with another embodiment. In the following description, the sameor similar constituent elements are denoted by the same referencenumerals. Also, redundant descriptions may be omitted.

First Embodiment

FIG. 3 schematically illustrates one example of the structure of animaging device 100. The imaging device 100 has a plurality of unitpixels 30 arranged in two dimensions. In practice, millions of unitpixels 30 are arranged in two dimensions. However, the unit pixels 30arranged in a matrix of 2 rows and 2 columns are illustrated in FIG. 3.The imaging device 100 may also be a line sensor. In such a case, theunit pixels 30 are arranged in one dimension (in a row direction or acolumn direction).

Each unit pixel 30 includes a first pixel cell 31 and a second pixelcell 31′. The first pixel cell 31 is a pixel cell corresponding to highsaturation. The second pixel cell 31′ is a pixel cell corresponding tolow noise. Typically, the first pixel cell 31 functions as a pixel cellfor low sensitivity, and the second pixel cell 31′ functions as a pixelcell for high sensitivity. The imaging device 100 has, for the firstpixel cells 31, reset signal lines 47 and address signal lines 48arranged for each row and vertical signal lines 45 and power-supplylines 46 arranged for each column. The imaging device 100 also has, forthe second pixel cells 31′, reset signal lines 47′ and address signallines 48′ arranged for respective rows and vertical signal lines 45′ andpower-supply lines 46′ arranged for respective columns.

The imaging device 100 has a first peripheral circuit and a secondperipheral circuit that are independent of each other. The firstperipheral circuit performs processing on signals from the first pixelcells 31, and the second peripheral circuit performs processing onsignals from the second pixel cells 31′. The first peripheral circuithas a first vertical scanning circuit 52, a first horizontal scanningcircuit 53, and first column analog-to-digital (AD) conversion circuits54. The second peripheral circuit has a second vertical scanning circuit52′, a second horizontal scanning circuit 53′, and second column ADconversion circuits 54′. However, the address signal lines 48 for thefirst pixel cells 31 and the address signal lines 48′ for the secondpixel cells 31′ can be shared, depending on the configuration of thepixels.

With respect to the first pixel cells 31, the first vertical scanningcircuit 52 controls the reset signal lines 47 and the address signallines 48. The vertical signal lines 45 are connected to the firsthorizontal scanning circuit 53 to transmit pixel signals to the firsthorizontal scanning circuit 53. The power-supply lines 46 supplypower-supply voltages to all of the corresponding unit pixels 30.

(Circuit Configuration of First Pixel cell 31 and Second Pixel cell 31′)

Next, an example of the circuit configuration of a first pixel cell 31and a second pixel cell 31′ will be described with reference to FIG. 4.

FIG. 4 is a circuit diagram of a unit pixel 30 and schematicallyillustrates the circuit configuration of a first pixel cell 31 and asecond pixel cell 31′. The first pixel cell 31 includes a firstphotoelectric converter PDS and a first charge detection circuit 51. Thesecond pixel cell 31′ includes a second photoelectric converter PDL anda second charge detection circuit 51′. The first photoelectric converterPDS and the second photoelectric converter PDL are light-receivingelements and are typically photodiodes (PDs). The first photoelectricconverter PDS may have a plane shape that is different from that of thesecond photoelectric converter PDL. In plan view, the area of the secondcharge detection circuit 51′ is larger than the area of the first chargedetection circuit 51.

In the first pixel cell 31 and the second pixel cell 31′, the firstphotoelectric converter PDS, which is provided in a semiconductorsubstrate, is arranged so as to be smaller than the second photoelectricconverter PDL, which is provided in a semiconductor substrate.Accordingly, compared with the first photoelectric converter PDS, thesecond photoelectric converter PDL generates a large amount of chargefor the same amount of incident light and thus has a high sensitivity.

In the first pixel cell 31, the first photoelectric converter PDS iselectrically connected to a capacitive element Csat, and a sourceelectrode of a reset transistor RSS and a gate electrode of anamplifying transistor SFS, which serves as an input of a source followercircuit, are connected to a node of the first photoelectric converterPDS and the capacitive element Csat. The reset transistor RSS resets(initializes) charge generated in the first photoelectric converter PDS.In other words, the reset transistor RSS resets a potential of the gateelectrode of the amplifying transistor SFS.

The first pixel cell 31 has a so-called three-transistor CMOS imagesensor pixel configuration. Heretofore, in a three-transistor pixelconfiguration, thermal noise, which is called reset noise, is generatedby an on-and-off operation of the reset transistor RSS. However, thefirst pixel cell 31 has a high saturation characteristic that canreceive a larger amount of light, by using the capacitive element Csatconnected to the first photoelectric converter PDS. When the amount oflight is large, optical shot noise is dominant in an acquired image.That is, since optical shot noise becomes larger than circuit noise, aninfluence of reset noise is small in the first pixel cell 31.

Thus, the first pixel cell 31 can function as a high saturation cell.Since the first pixel cell 31 does not require a transfer transistor,which is required by a known CMOS image sensor, space is correspondinglyfreed in the silicon substrate. As a result, by using the space, it ispossible to ensure the area of the second photoelectric converter PDL inthe second pixel cell 31′.

The second pixel cell 31′ has the second photoelectric converter PDL, atransfer transistor TX, and a floating diffusion FD. The secondphotoelectric converter PDL is connected to the floating diffusion(hereinafter referred to simply as “FD”) via the transfer transistor TX.The second pixel cell 31′ has a so-called four-transistor pixel circuitconfiguration. Charge generated by the second photoelectric converterPDL is fully transferred to the FD via the transfer transistor TX, andthus, through a correlated double sampling (CDS) operation, noisesubtraction can be performed on noise generated by a reset transistorRSL.

As described above, the first pixel cell 31 is made to have lowsensitivity characteristic by reducing the area of the photodiode. Thefirst pixel cell 31 is also made to have high saturation characteristicby including the capacitive element Csat in the wiring layer. Sincereducing noise is not so highly requested, it is possible to increasethe area of the second photoelectric converter PDL in the second pixelcell 31′ by reducing the number of elements, such as transistors fornoise reduction, fabricated using a silicon substrate.

Since the second pixel cell 31′ is a cell for high sensitivity, the areaof the photodiode is sufficiently reserved. In addition, when a knowntransistor configuration is employed, the second pixel cell 31′ can havea low noise characteristic.

The first pixel cell 31 images a high-luminance subject, andsimultaneously, the second pixel cell 31′ images a low-luminancesubject. This makes it possible to achieve a wide dynamic range whilecompletely simultaneously performing imaging.

The circuit configuration of the unit pixel 30 will be described belowwith reference FIGS. 3 and 4 while paying particular attention to thefirst pixel cell 31.

The first charge detection circuit 51 includes the amplifying transistorSFS, the reset transistor RSS, and an address transistor SELS.

The first photoelectric converter PDS is electrically connected to asource electrode of the reset transistor RSS and a gate electrode of theamplifying transistor SFS. The first photoelectric converter PDSconverts light (incident light) incident on the first pixel cell 31 intocharge. The first photoelectric converter PDS generates signal chargecorresponding to the amount of incident light. The generated signalcharge is stored by a charge storage node 44.

The power-supply line 46 is connected to a drain electrode of theamplifying transistor SFS. The power-supply lines 46 are arranged in acolumn direction. This is due to the following reason. The first pixelcells 31 are selected for each row. Thus, when the power-supply lines 46are arranged in a row direction, driving currents for all the pixelcells of one row flow to one power-supply line 46, and a large drop involtage may occurs. A common source follower power-supply voltage isapplied to the amplifying transistors SFS in all the first pixel cells31 through the power-supply lines 46 in the imaging device 100.

The amplifying transistor SFS amplifies a signal voltage correspondingto the amount of the signal charge stored in the corresponding chargestorage node 44. A gate electrode of the address transistor SELS isconnected to the first vertical scanning circuit 52 through the addresssignal line 48. A drain electrode of the address transistor SELS isconnected to the first horizontal scanning circuit 53 through verticalsignal line VSIGS. The vertical signal lines VSIGS and VSIGL correspondto the vertical signal lines 45 and 45′, illustrated in FIG. 3,respectively. The address transistor SELS selectively outputs thevoltage, which the amplifying transistor SFS outputs, to the verticalsignal line VSIGS.

The first vertical scanning circuit 52 applies row selection signals forcontrolling on and off operations of the address transistors SELS to thegate electrodes of the address transistors SELS. Thus, a row to be readis scanned in a vertical direction (column direction) and is selected.Signal voltage is read out from the first pixel cell 31 in the unitpixel 30 in the selected row to the corresponding vertical signal lineVSIGS. Also, the first vertical scanning circuit 52 applies resetsignals for controlling on and off operations of the reset transistorsRSS to the gate electrodes of the reset transistors RSS. Thus, the firstpixel cells 31 in the unit pixels 30 in a row subject to reset operationare selected.

Each first column AD conversion circuit 54 performs, for example, noisesuppression signal processing and analog-to-digital conversion (ADconversion), typified by correlated double sampling, on the signals readout from the first pixel cells 31 to the vertical signal line VSIGS foreach row. The first horizontal scanning circuit 53 reads the signalsprocessed by the first column AD conversion circuit 54.

In the imaging device 100, random noise may generate during transfer orreset of the signal charge. However, a description herein will be givenassuming that the reset noise that generates during reset of the signalcharge is random noise. When random noise remains during the resetoperation, the remaining random noise is then added to signal chargestored in the charge storage node 44. In this case, when the signalcharge is read out, a signal containing random noise is output.

(Device Structure of Unit Pixel 30)

FIG. 5 schematically illustrates a cross section of the device structureof each unit pixel 30 in the imaging device 100 according to the presentembodiment.

In the unit pixel 30, the first pixel cell 31 and the second pixel cell31′ are arranged adjacent to each other. The unit pixel 30 typically hasan N-type silicon substrate 300 including the first photoelectricconverter PDS and the second photoelectric converter PDL, a color filter305, and microlenses 302A and 302B. The unit pixel 30 includes the firstpixel cell 31 and the second pixel cell 31′. However, when monochromeimaging is only performed, the color filter 305 may be eliminated. Also,when light-collecting using microlenses is not performed, themicrolenses 302A and 302B may be eliminated. The first photoelectricconverter PDS and the second photoelectric converter PDL are generallyformed by implanting impurities into a silicon substrate, and the depthand width thereof are not limited to those illustrated in FIG. 5.

In the present embodiment, the sensitivity of the first pixel cell 31 islower than the sensitivity of the second pixel cell 31′. The microlens302A entirely covers the first photoelectric converter PDS. Themicrolens 302B entirely covers the second photoelectric converter PDL.In plan view, the area of the first photoelectric converter PDS isdifferent from the area of the second photoelectric converter PDL. Morespecifically, the area of the first photoelectric converter PDS issmaller than the area of the second photoelectric converter PDL.

FIGS. 6 and 7 schematically illustrate a cross section of another devicestructure of each unit pixel 30 in the imaging device 100 according tothe present embodiment. As illustrated in FIG. 6, the first pixel cell31 and the second pixel cell 31′ may have a common microlens 302. Themicrolens 302 focuses light incident on the unit pixel 30 onto thecorresponding photoelectric converter. As illustrated in FIG. 7, themicrolens 302 may be provided only for the second pixel cell 31′, whichis a cell for high sensitivity. The second photoelectric converter PDLmay be located on an optical axis of the microlens 302.

The first photoelectric converter PDS and the second photoelectricconverter PDL may be separated from each other by a shallow trenchisolation (STI) layer 303 formed in a silicon substrate. Thiselectrically reduces mixing of colors. However, a configuration thatdoes not have the STI layer 303 can also be selected depending on apurpose, such as miniaturization or the like.

In the present embodiment, difference in sensitivity is caused bydifference in size between the first photoelectric converter PDS and thesecond photoelectric converter PDL. Also, the capacitive element Csatformed in a wiring layer 301 is electrically connected to the chargestorage node 44 (see FIG. 4) in the first pixel cell 31 via a contact304. When the charge storage capacitance is increased by the capacitiveelement Csat, it is possible to increase the number of saturationelectrons in the first pixel cell 31. The first pixel cell 31 functionsas a pixel cell corresponding to high saturation. This makes it possibleto obtain higher saturation charge at low sensitivity. In other words,it is possible to image a high-luminance subject without saturation. Inthe present disclosure, the “storage capacitance” refers to allcapacitance components connected to a photoelectric converter.

In plan view, the capacitive element Csat is located between the firstphotoelectric converter PDS and the second photoelectric converter PDL.In plan view, the capacitive element Csat at least partly overlaps oneof or both the first photoelectric converter PDS and the secondphotoelectric converter PDL. The capacitive element Csat may beimplemented by a metal insulator metal (MIM) capacitor, which has aparallel-plate capacitor configuration between different wiring layers,as illustrated in FIG. 5. In such a case, the capacitive element Csatincludes a lower electrode 311, an upper electrode 310, and an insulator312 provided between the lower electrode 311 and the upper electrode310. One of the lower electrode 311 and the upper electrode 310 iselectrically connected to the first photoelectric converter PDS.

FIG. 8 schematically illustrates a cross section of another devicestructure of each unit pixel 30 in the imaging device 100 according tothe present embodiment. The capacitive element Csat may be implementedby a metal oxide metal (MOM) capacitor, which forms a capacitancebetween the same-layer wiring lines, as illustrated in FIG. 6. Inaddition, a depletion metal oxide semiconductor (DMOS) capacitor using asilicon substrate 300 can also be selected as the capacitive elementCsat.

The device structures illustrated in FIGS. 6 to 8 are generally calledback-side illumination (BSI) structures. The backside illuminationstructures have advantages that the wiring line area can be used as acapacitor and a high aperture ratio can be obtained even when thecapacitive element Csat is provided.

FIG. 9 schematically illustrates a cross section of yet another devicestructure of each unit pixel 30 in the imaging device 100 according tothe present embodiment. The illustrated device structure is generallycalled a front-side illumination (FSI) structure. In this structure, aphotoelectric conversion layer is provided at an obverse side of thesilicon substrate 300, and incident light from the obverse side isdetected. The imaging device in the present disclosure also encompassesfront-side illumination device structures.

FIG. 10 schematically illustrates a layout example of the unit pixels 30in the imaging device 100 according to the present embodiment whenviewed from a bird's eye. FIG. 10 illustrates the unit pixels 30 in 3rows and 3 columns. FIG. 5 or 8 schematically illustrates a crosssection of the unit pixels 30 along line V, VIII-V, VIII illustrated inFIG. 10. On-chip microlenses 302A are configured so as to focus lightonto the corresponding first photoelectric converters PDS. On-chipmicrolenses 302B are configured so as to focus light onto thecorresponding second photoelectric converters PDL. The light-collectingarea of each on-chip microlens 302A is larger than the light-collectingarea of each on-chip microlens 302B. As illustrated in FIG. 7, aconfiguration in which each first pixel cell 31 has a lower sensitivitymay be selected by eliminating the microlens 302A for the firstphotoelectric converter PDS. Also, the light-collecting characteristicmay be improved by arranging a common microlens for the firstphotoelectric converter PDS and the second photoelectric converter PDLand by increasing the pitch of the microlenses, as illustrated in FIG.6.

A wide variety of materials that are generally used to manufacturesilicon semiconductor devices may be used as materials of the unitpixels 30.

Variations of the circuit configuration of the unit pixel 30 will bedescribed below with reference to FIGS. 11 to 21.

FIGS. 11 to 22 schematically illustrate variations of the circuitconfiguration of each unit pixel 30 (specifically, the circuitconfiguration of each pixel cell) according to the present embodiment.As illustrated in FIGS. 11 to 22, the circuit configuration of each unitpixel 30 according to the present embodiment has variations. In additionto the illustrated configurations, for example, some of the variationscan also be combined together.

(First Variation)

FIG. 11 illustrates a first variation of the circuit configuration ofeach unit pixel 30. Unlike the configuration of the first pixel cell 31illustrated in FIG. 4, the first pixel cell 31 does not have, as acharge storage capacitance, the capacitive element Csat connected to thecharge storage node 44. The first pixel cell 31 is athree-transistor-type cell constituted by the reset transistor RSS, theamplifying transistor SFS, the address transistor SELS, and the firstphotoelectric converter PDS. The configuration of the second pixel cell31′ is the same as the configuration illustrated in FIG. 4.

According to the first variation, a parasitic capacitance viewed fromthe source electrode of the reset transistor RSS and a gate capacitanceof the amplifying transistor accompany the first photoelectric converterPDS. Thus, it is possible to use the parasitically accompanyingcapacities, instead of additionally providing a capacitive element.

(Second Variation)

FIG. 12 illustrates a second variation of the circuit configuration ofeach unit pixel 30. Unlike the configuration of the first pixel cell 31illustrated in FIG. 4, the first pixel cell 31 further includes afeedback loop (a column feedback circuit). The column feedback circuitincludes the amplifying transistor SFS, the address transistor SELS, aninverting amplifier circuit FBAMP1, and the reset transistor RSS. Thecolumn feedback circuit performs feedback to reset the first pixel cell31. The configuration of the second pixel cell 31′ is the same as theconfiguration illustrated in FIG. 4.

In the first pixel cell 31, during a reset operation, the resettransistor RSS is turned on to fix the charge storage node 44 to avoltage of the drain electrode of the reset transistor RSS. The chargestorage node 44 is connected to the gate electrode of the amplifyingtransistor SFS, and a signal voltage of the charge storage node 44 isoutput to the vertical signal line VSIGS via the address transistor SELSthat is turned on. The signal output to the vertical signal line VSIGSis input to the first inverting amplifier circuit FBAMP1 in the columnfeedback circuit provided in the corresponding column. A voltage towhich a negative gain is applied in the first inverting amplifiercircuit FBAMP1 is applied to the drain electrode of the reset transistorRSS through a column feedback signal line FBS.

According to the second variation, reset noise, which is fluctuation ofa reset voltage of the charge storage node 44, can be reduced bynegative feedback. In addition, in a backside illumination sensor likethat illustrated in FIG. 5, the transfer transistor TX does not need tobe formed in the silicon substrate 300, so that the aperture ratio canbe increased correspondingly.

A tapered reset system in which a tapered voltage, that is, a voltagethat increases or decreases gradually with time, is applied to the gateof the reset transistor RSS can also be employed during negativefeedback. A drive scheme that is generally used in order to reduce resetnoise of a three-transistor CMOS image sensor can be used. One exampleof the drive scheme is a flash reset system that is a combination ofstrong inversion reset and weak inversion reset.

(Third Variation)

FIG. 13 illustrates a third variation of the circuit configuration ofeach unit pixel 30. Unlike the configuration of the first pixel cell 31illustrated in FIG. 4, the first pixel cell 31 includes a transfertransistor TXS. The configuration of the second pixel cell 31′ is thesame as the configuration illustrated in FIG. 4. In this configuration,each of the first pixel cell 31 and the second pixel cell 31′ furtherhas the transfer transistor and thus has a four-transistor-typeconfiguration. Only the first pixel cell 31 has the capacitive elementCsat for high saturation.

According to the third variation, charges generated in all pixel cellsis temporarily transferred to a charge holding portion, that is, tofloating diffusions FDS and FDL, thereby making it possible to realize aglobal shutter operation.

The first pixel cell 31 may further include a column feedback circuit,as in the second variation. That is, a configuration that reduces resetnoise may be employed by providing the feedback circuit illustrated inFIG. 12 for each of the reset transistors RSS and RSL.

(Fourth Variation)

FIG. 14 illustrates a fourth variation of the circuit configuration ofeach unit pixel 30. Unlike the circuit configuration illustrated in FIG.4, each of the first pixel cell 31 and the second pixel cell 31′ has athree-transistor-type configuration and has a column feedback circuitincluding an inverting amplifier circuit FBAMPS or an invertingamplifier circuit FBAMPL.

According to the fourth variation, since neither the first pixel cell 31nor the second pixel cell 31′ has a transfer transistor, it is possibleto further increase the area of the second photoelectric converter PDLin the second pixel cell 31′, which requires a high sensitivity.

(Fifth and Sixth Variations)

FIG. 15 illustrates a fifth variation of the circuit configuration ofeach unit pixel 30. The configuration of the first pixel cell 31 in thefifth variation differs from the configuration of the first pixel cell31 illustrated in FIG. 12. That is, the first pixel cell 31 in the fifthvariation further includes a capacitive element Cc, a capacitive elementCs, and a feedback control transistor FBS. It is desirable that thecapacitance value of the capacitive element Cc be smaller than thecapacitance value of the capacitive element Cs. The first pixel cell 31does not have the capacitive element Csat illustrated in FIG. 12.

FIG. 16 illustrates a modification of the fifth variation of the circuitconfiguration of each unit pixel 30. In the modification of the fifthvariation, the capacitive element Csat is connected to the firstphotoelectric converter PDS.

According to the fifth variation and the modification thereof, noise canbe attenuated in accordance with the ratio of the capacitance value ofthe capacitive element Cs to the capacitance value of the capacitiveelement Cc. As a result, the advantage of the reset noise reduction canbe expected, compared with the configuration illustrated in FIG. 12.

FIG. 17 illustrates a sixth variation of the circuit configuration ofeach unit pixel 30. In the sixth variation, an element to which thesource or drain of the reset transistor RSS is connected differs fromthat in the fifth variation. An advantage that is the same as that inthe fifth variation can also be expected in the sixth variation.

(Seventh and Eighth Variations)

FIG. 18 illustrates a seventh variation of the circuit configuration ofeach unit pixel 30. Compared with the configuration illustrated in FIG.15, the first pixel cell 31 has an in-pixel feedback circuit thatperforms negative feedback in the pixel. The in-pixel feedback circuitincludes the amplifying transistor SFS, the feedback control transistorFBS, the capacitive element Cs, and the capacitive element Cc. Aplurality of reference voltages is applied to a drain VB10 of theamplifying transistor SFS in accordance with an operation mode.

According to the seventh variation, noise can be attenuated inaccordance with the ratio of the capacitance value of the capacitiveelement Cs to the capacitance value of the capacitive element Cc, andalso high-speed drive can be performed since speed reduction when acolumn feedback circuit is used does not occur.

FIG. 19 illustrates an eighth variation of the circuit configuration ofeach unit pixel 30. In the eighth variation, an element to which thesource or drain of the reset transistor RSS is connected differs fromthat in the seventh variation. According to the eighth variation, ahigh-speed operation and reset noise reduction using in-pixel feedbackreset can be realized, as in the seventh variation.

(Ninth and Tenth Variations)

FIG. 20 illustrates a ninth variation of the circuit configuration ofeach unit pixel 30. Compared with the configuration illustrated in FIG.4, the second pixel cell 31′ also has a capacitive element CsatLconnected to the second photoelectric converter PDL, as in the firstpixel cell 31. A signal line VPUMP is connected to the capacitiveelement CsatL.

According to the ninth variation, application of a pulse voltage to thesignal line VPUMP makes it possible to increase the voltage level of thesecond photoelectric converter PDL in the high-sensitivity cell via thecapacitive element CsatL. As a result, a sufficient signal range can beensured even during low-voltage operation. In addition, the second pixelcell 31′, which is a high-sensitivity cell, may have a feedback circuit.In such a case, it is possible to perform low-noise operation throughreduction of reset noise, in addition to the low-voltage operation. Whenthe feedback circuit has a plurality of capacitive elements, resistanceelements, and transistor elements, in addition to the reset transistorRSL, it is possible to perform negative feedback with a higher gain.

FIG. 21 illustrates a tenth variation of the circuit configuration ofeach unit pixel 30. The second pixel cell 31′, which is ahigh-sensitivity cell, may have a column feedback circuit. According tothis configuration, higher sensitivity can be realized through selectivereduction of noise in the second pixel cell 31′. A reset system in thiscase may be the high-gain column feedback reset system described abovewith reference to FIG. 16 or 17 or the in-pixel feedback reset systemdescribed above with reference to FIG. 18 or 19. With regard to theabove described variations, details of the reduction of reset noise byusing feedback are described in International Publication No.2012/147302 and U. S. Unexamined Patent Application Publication No.2016/0190187. The contents of International Publication No. 2012/147302and U. S. Unexamined Patent Application Publication No. 2016/0190187 areincorporated herein by reference in their entirety.

(11th Variation)

FIG. 22 illustrates an 11th variation of the circuit configuration ofeach unit pixel 30. The first pixel cell 31 and the second pixel cell31′ share a charge detection circuit that has an amplifying transistorSFL and an address transistor SELL. The first pixel cell 31 and thesecond pixel cell 31′ share the reset transistor RSL as a transistor forreset. The transfer transistors TXS are TXL are used to select which ofthe first pixel cell 31 and the second pixel cell 31′ is to be reset orread out.

According to the 11th variation, it is possible to reduce the number oftransistors used in the entire unit pixel 30. As a result, it ispossible to increase the area of the second photoelectric converter PDLin the unit pixel 30.

(Drive Method for Imaging Device 100)

One example of an operation sequence of the imaging device 100 will nowbe described with reference to FIG. 23.

FIG. 23 schematically illustrates exposure and a reading operation inone cycle (one frame) period in the imaging device 100. The horizontalaxis represents time, and the vertical axis represents a row to be readout. FIG. 23 illustrates the state of the so-called rolling shutterreadout. In the imaging device 100, when the first pixel cell 31 and thesecond pixel cell 31′ are used to perform exposure and a readoutoperation at the same timing, the dynamic range can be increased.

In the device configuration illustrated in FIG. 5, difference insensitivity of about one digit occurs between the first pixel cell 31and the second pixel cell 31′. Thus, even when the same exposure andreadout are performed, the dynamic range can be increased by about onedigit compared to a general pixel.

In the present embodiment, in order to further increase the dynamicrange, each of the first pixel cell 31 and the second pixel cell 31′ hasindependent exposure and readout timings. In one cycle of an imagingoperation, the second pixel cell 31′ performs exposure in a firststorage time T1, and the first pixel cell 31 performs exposure in secondstorage times T2 and T3, which are shorter than the first storage timeT1. A specific description will be given below.

In the present embodiment, for example, one cycle is 1/60th of a second.First, the second pixel cells 31′ performs exposure in the storage timeT1, which is close to one cycle, and after the storage time passes,charges in the second pixel cells 31′ are sequentially read out for eachrow (readout 1). When the readout for each row is completed, chargesstored in all the second pixel cells 31′ in the read row are reset.

In the first pixel cells 31, non-destructive readout is performed atleast twice in one cycle. For example, first exposure is performed inthe storage time T2, which is 1/30th of one cycle period (i.e., 1/1800thof a second), and after the exposure is completed, readout (readout 2)is performed. Thereafter, second exposure is performed in the storagetime T3, which is one-half of one cycle period (that is, 1/120th of asecond), without performing resetting of the stored charges, and afterthe exposure is completed, readout (readout 3) is performed. In such anoperation sequence, three pieces of imaging data exposure times of whichare different from each other can be obtained in one cycle period.Although the dynamic range can be improved by about 1 digit when thesame exposure and readout are performed, as described above, combiningthe pieces of imaging data makes it possible to generate an image with adynamic range that is additionally higher by about 1.5 digits, that is,an image with a dynamic range that is higher by a total of about 2.5digits compared to a general pixel.

As described above, the first pixel cell 31 functions as an imagingregion that images a bright subject, which has a large amount of light.A desirable characteristic requested for the first pixel cell 31 is thatthe number of saturation electrons is large (i.e., the saturation ishigh). On the other hand, the second pixel cell 31′ functions as animaging region that images a dark subject, which has a small amount oflight. A desirable characteristic requested for the second pixel cell31′ is that the amount of random noise is small. The second pixel cell31′ may have a small number of saturation electrons, that is, may be lowin the saturation. According to the present embodiment, it is possibleto provide the imaging device 100 that can satisfy the above-describedcharacteristics.

Second Embodiment

An imaging module 200 according to the present embodiment will bedescribed with reference to FIG. 24.

FIG. 24 schematically illustrates functional blocks in the imagingmodule 200 including the imaging device 100.

The imaging module 200 has the imaging device 100 according to the firstembodiment and a digital signal processor (DSP) 400. The imaging module200 processes signals obtained by the imaging device 100 and outputs theprocessed signals to outside.

The DSP 400 functions as a signal processing circuit that processes thesignals output from the imaging device 100. That is, the DSP 400receives digital pixel signals output from the imaging device 100. TheDSP 400 performs processing, for example, gamma correction processing,color interpolation processing, space interpolation processing, andautomatic white balance processing. The DSP 400 may be a microcomputerthat controls the imaging device 100 in accordance with various settingsspecified by a user and that integrates operations of the entire imagingmodule 200.

The DSP 400 processes digital pixel signals output from the imagingdevice 100 to determine optimum reset voltages (VRG, VRB, and VRR). TheDSP 400 feeds back the reset voltages to the imaging device 100. Herein,VRG, VRB, and VRR indicate a reset voltage for green (G) pixels, a resetvoltage for blue (B) pixels, and a reset voltage for red (R) pixels,respectively. The reset voltages may be feedback signals transmittedfrom the feedback signal lines FBS or the vertical signal lines 45. Theimaging device 100 and the DSP 400 can also be manufactured as onesemiconductor device (the so-called System on a Chip (SoC)). This makesit possible to miniaturize electronic equipment using the imaging device100.

Naturally, it is also possible to put only the imaging device 100 intoproduction without incorporating it into a module. In such a case, asignal processing circuit may be externally connected to the imagingdevice 100 to perform signal processing outside the imaging device 100.

The imaging device according to the present disclosure is useful forimage sensors used in cameras, for example, digital cameras andvehicle-mounted cameras.

The imaging device according to the present disclosure is applicable tovarious sensor systems and camera systems, such as digital stillcameras, medical cameras, camera for monitoring, vehicle-mountedcameras, digital single-lens reflex cameras, and digital mirrorlessinterchangeable lens cameras.

What is claimed is:
 1. An imaging device comprising: a semiconductorsubstrate; a first pixel cell including a first photoelectric converterin the semiconductor substrate, and a first capacitive element one endof which is electrically connected to the first photoelectric converter;and a second pixel cell including a second photoelectric converter inthe semiconductor substrate, wherein, an area of the secondphotoelectric converter is larger than an area of the firstphotoelectric converter in a plan view, a number of charges generated inthe second pixel cell is greater than a number of charges generated inthe first pixel cell when the first and second pixel cells receiveincident light, and a number of saturation charges of the first pixelcell is greater than a number of saturation charges of the second pixelcell.
 2. The imaging device according to claim 1, wherein the firstpixel cell further includes a first charge detection circuitelectrically connected to the first photoelectric converter; and thesecond pixel cell further includes a second charge detection circuitelectrically connected to the second photoelectric converter.
 3. Theimaging device according to claim 2, wherein the first charge detectioncircuit includes a first reset transistor one of a source and a drain ofwhich is electrically connected to the first photoelectric converter;and the second charge detection circuit includes a second resettransistor one of a source and a drain of which is electricallyconnected to the second photoelectric converter.
 4. The imaging deviceaccording to claim 2, wherein the first pixel cell includes a firsttransfer transistor one of a source and a drain of which is electricallyconnected to the first photoelectric converter; and the first chargedetection circuit includes a first floating diffusion electricallyconnected to the other of the source and the drain of the first transfertransistor, and a first reset transistor one of a source and a drain ofwhich is electrically connected to first floating diffusion.
 5. Theimaging device according to claim 2, wherein the second pixel cellincludes a second transfer transistor one of a source and a drain ofwhich is electrically connected to the second photoelectric converter;and the second charge detection circuit includes a second floatingdiffusion electrically connected to the other of the source and thedrain of the second transfer transistor, and a second reset transistorone of a source and a drain of which is electrically connected to thesecond floating diffusion.
 6. The imaging device according to claim 1,wherein the second pixel cell includes a transfer transistor one of asource and a drain of which is electrically connected to the secondphotoelectric converter; and a floating diffusion electrically connectedto the other of the source and the drain of the transfer transistor. 7.The imaging device according to claim 1, wherein the first pixel cellfurther includes a first transfer transistor one of a source and a drainof which is electrically connected to the first photoelectric converter;the second pixel cell further includes a second transfer transistor oneof a source and a drain of which is electrically connected to the secondphotoelectric converter, and a floating diffusion electrically connectedto the other of the source and the drain of the second transfertransistor; and the floating diffusion is electrically connected to theother of the source and the drain of the first transfer transistor. 8.The imaging device according to claim 7, wherein the second pixel cellfurther includes a reset transistor one of a source and a drain of whichis electrically connected to the floating diffusion.
 9. The imagingdevice according to claim 1, wherein the second pixel cell does not havea capacitive element.
 10. The imaging device according to claim 1,wherein a shape of the second photoelectric converter is different froma shape of the first photoelectric converter in the plan view.
 11. Theimaging device according to claim 1, wherein the first pixel cellfurther includes a first microlens located at a light incident side ofthe first photoelectric converter; the second pixel cell furtherincludes a second microlens located at a light incident side of thesecond photoelectric converter; and a light-collecting area of thesecond microlens is larger than a light-collecting area of the firstmicrolens.
 12. The imaging device according to claim 1, wherein thefirst pixel cell and the second pixel cell directly adjoin each other;the first pixel cell and the second pixel cell further include a commonmicrolens located at a light incident side of the first photoelectricconverter and the second photoelectric converter; and the secondphotoelectric converter is located on an optical axis of the commonmicrolens.
 13. The imaging device according to claim 1, wherein thefirst capacitive element includes a first electrode, a second electrode,and an insulator between the first electrode and the second electrode,and one of the first electrode and the second electrode is electricallyconnected to the first photoelectric converter.
 14. The imaging deviceaccording to claim 1, wherein a second storage time when the secondpixel cell accumulates a second charge generated in the secondphotoelectric converter is longer than a first storage time when thefirst pixel cell accumulates a first charge generated in the firstphotoelectric converter.
 15. The imaging device according to claim 2,wherein the first charge detection circuit detects the first charge atleast twice without resetting the first charge, during one frame period.16. An imaging device comprising: a semiconductor substrate; a firstpixel cell including a first photoelectric converter that is located inthe semiconductor substrate and converts incident light into a firstcharge, a first storage capacitance that is electrically connected tothe first photoelectric converter and accumulates at least a part of thefirst charge, and a first charge detection circuit that is connected tothe first storage capacitance and detects the first charge; and a secondpixel cell including a second photoelectric converter that is located inthe semiconductor substrate and converts incident light into a secondcharge, a second storage capacitance that is connected to the secondphotoelectric converter and accumulates at least a part of the secondcharge, and a second charge detection circuit that is connected to thesecond storage capacitance and detects the second charge, wherein anarea of the second photoelectric converter is larger than an area of thefirst photoelectric converter in a plan view, and a capacitance value ofthe first storage capacitance is larger than a capacitance value of thesecond storage capacitance, a number of charges generated in the secondpixel cell is greater than a number of charges generated in the firstpixel cell when the first and second pixel cells receive incident light,and a number of saturation charges of the first pixel cell is greaterthan a number of saturation charges of the second pixel cell.
 17. Theimaging device according to claim 16, wherein the first storagecapacitance includes a capacitive element, and the second storagecapacitance does not include a capacitive element.
 18. The imagingdevice according to claim 16, wherein the first pixel cell and thesecond pixel cell directly adjoin each other; the first pixel cell andthe second pixel cell further include a common microlens located atlight-incident side of the first photoelectric converter and the secondphotoelectric converter; and the second photoelectric converter islocated on an optical axis of the common microlens.
 19. An imagingdevice comprising: a semiconductor substrate; a first pixel cellincluding a first photoelectric converter in the semiconductorsubstrate, and a first capacitive element one end of which iselectrically connected to the first photoelectric converter; and asecond pixel cell including a second photoelectric converter in thesemiconductor substrate, wherein, an area of the second photoelectricconverter is larger than an area of the first photoelectric converter ina plan view, the first pixel cell and the second pixel cell directlyadjoin each other, the semiconductor substrate includes a region betweenthe first photoelectric converter and the second photoelectricconverter, and the first capacitive element at least partly overlaps, inthe plan view, with the region.
 20. The imaging device according toclaim 19, wherein the first capacitive element at least partly overlaps,in the plan view, with the second photoelectric converter.
 21. Theimaging device according to claim 1, wherein the first pixel cell andthe second pixel cell include a charge detection circuit, the chargedetection circuit includes a floating diffusion, a reset transistor oneof a source and a drain of which is electrically connected to thefloating diffusion, and an amplification transistor having a gateelectrically connected to the floating diffusion, the first pixel cellfurther includes a first transfer transistor, the first photoelectricconverter being electrically connected to the floating diffusion via thefirst transfer transistor, and the second pixel cell further includes asecond transfer transistor, the second photoelectric converter beingelectrically connected to the floating diffusion via the second transfertransistor.